The present invention relates to an improvement in a method of fixing a member in a ceramic body and a ceramic body with a member fixed in it by the method, and more particularly, to a method of fixing the member in the ceramic body when the thermal expansion coefficient of the ceramic body is smaller than that of the member.
Examples of products manufactured by sealing rodlike articles (to be referred to as rods hereinafter) in ceramic articles (to be referred to as ceramic bodies hereinafter), that is, products in which rods close holes of ceramic bodies and are mounted therein, are (1) lead electrodes mounted and extending through the wall of a ceramic vacuum tube, (2) electrodes mounted and extending through the wall of a ceramic sample container in a conductimeter meter, and (3) electrodes mounted and extending through the wall of a ceramic measuring tube in an electromagnetic flowmeter.
A conductive rod is fixed in a ceramic body according to the following steps of a conventional technique: (1) A nonsintered ceramic body made of, for example, magnesia (MgO) is prepared. The ceramic body is a soft material prepared by pressing a mixture of, for example, magnesia micropowder and a plastic binder; (2) A hole is formed in the ceramic body; (3) The ceramic body is sintered at a relatively low temperature (generally 200.degree..about.300.degree. C.) for dewaxing the binder included in the ceramic body; (4) A rod made of, for example, platinum is inserted in the hole; (5) The ceramic body and the rod are sintered at a sintering temperature (generally, 1,000.degree. C. to 1,500.degree. C.) determined by the kind of ceramic material. The ceramic body shrinks at a line shrinkage coefficient of 16% (corresponding to a volume shrinkage coefficient of 41%) to 26% (corresponding to a volume shrinkage coefficient of 60%) when the ceramic body is sintered. Therefore, no gap is formed between the rod and the hole of the ceramic body at a high temperature after sintering; and (6) When the temperature of the sintered body is cooled to room temperature, the ceramic body shrinks greater than the rod since the thermal expansion coefficient of ceramic is bigger than that of the rod. As a result, the ceramic body tightens the rod with a large force, and the rod closes the hole of the ceramic body and fixed therein.
In the above process, magnesia or forsterite is used as ceramic, and platinum is used as the rod material. Therefore the thermal expansion coefficient of the ceramic body is larger than that of the rod, no problem is presented. However, if alumina (Al.sub.2 O.sub.3) or spinel (MgO.Al.sub.2 O.sub.3) having a better anti-corrosion property than that of magnesia or forsterite is used, the following disadvantage is presented. Thermal expansion coefficient .alpha. of alumina (Al.sub.2 O.sub.3) or spinel (MgO.Al.sub.2 O.sub.3) is 7.1 to 7.3.times.10.sup.-6 and is smaller than thermal expansion coefficient .beta.=8.9.times.10.sup.-6 of platinum. For this reason, if a platinum rod is fixed in an alumina ceramic body by the above-mentioned conventional method, the rod shrinks greater than the wall surface defining the hole. A gap is formed between the rod and the wall surface defining the hole, so the rod cannot be fixed in the ceramic body, and an air-tight state between the hole and the rod cannot be guaranteed.